Ultracapacitor electroyte

ABSTRACT

The invention relates to an ultracapacitor and to a method of making an ultracapacitor. The ultracapacitor of the invention includes two solid, nonporous current collectors, two porous electrodes separating the collectors, a porous separator between the electrodes and an electrolyte occupying the pores in the electrodes and separator. The electrolyte includes a cyclic carbonate solvent, a cyclic ester solvent and an electrolyte salt. The invention also relates to a stack of ultracapacitor cells.

This invention was made with government support under Contract No.38-83CH10093 awarded by DOE. The government may have certain rights inthe invention.

BACKGROUND OF THE INVENTION

Capacitors are storage devices that store electrical energy on anelectrode surface. Electrochemical cells create an electrical charge atelectrodes by chemical reaction. The ability to store or createelectrical charge is a function of electrode surface area in bothapplications. Ultracapacitors, sometimes referred to as double layercapacitors, are a third type of storage device. An ultracapacitorcreates and stores energy by microscopic charge separation at anelectrical chemical interface between electrode and electrolyte.

Ultracapacitors are able to store more energy per weight thantraditional capacitors and they typically deliver the energy at a higherpower rating than many rechargeable batteries. Ultracapacitors comprisetwo porous electrodes that are isolated from electrical contact by aporous separator. The separator and the electrodes are impregnated withan electrolytic solution, which allows ionic current to flow between theelectrodes while preventing electronic current from discharging thecell. Each electrode is in intimate contact with a current collector.One purpose of the current collector is to reduce ohmic loss. If thecurrent collectors are nonporous, they can also be used as part of thecapacitor case and seal.

When electric potential is applied to an ultracapacitor cell, ioniccurrent flows due to the attraction of anions to the positive electrodeand cations to the negative electrode. Upon reaching the electrodesurface, the ionic charge accumulates to create a layer at the solidliquid interface region. This is accomplished by absorption of thecharge species themselves and by realignment of dipoles of the solventmolecule. The absorbed charge is held in this region by opposite chargesin the solid electrode to generate an electrode potential. Thispotential increases in a generally linear fashion with the quantity ofcharge species or ions stored on the electrode surfaces. Duringdischarge, the electrode potential or voltage that exists across theultracapacitor electrodes causes ionic current to flow as anions aredischarged from the surface of the positive electrode and cations aredischarged from the surface of the negative electrode while anelectronic current flows through an external circuit between electrodecurrent collectors.

In summary, the ultracapacitor stores energy by separation of positiveand negative charges at the interface between electrode and electrolyte.An electrical double layer at this location consists of sorbed ions onthe electrode as well as solvated ions. Proximity between the electrodesand solvated ions is limited by a separation sheath to create positiveand negative charges separated by a distance which produces a truecapacitance in the electrical sense.

During use, an ultracapacitor cell is discharged by connecting theelectrical connectors to an electrical device such as a portable radio,an electric motor, light emitting diode or other electrical device. Theultracapacitor is not a primary cell but can be recharged. The processof charging and discharging may be repeated over and over. For example,after discharging an ultracapacitor by powering an electrical device,the ultracapacitor can be recharged by supplying potential to theconnectors.

The physical processes involved in energy storage in an ultracapacitorare distinctly different from the electrochemical oxidation/reductionprocesses responsible for charge storage in batteries. Further unlikeparallel plate capacitors, ultracapacitors store charge at an atomiclevel between electrode and electrolyte. The double layer charge storagemechanism of an ultracapacitor is highly efficient and can produce highspecific capacitance, up to several hundred Farads per cubic centimeter.

Ultracapacitor electrolytes must exhibit good ionic conductivity andmust be highly soluble and chemically and electrochemically stable.Additionally, the electrolyte must be low in cost for commercialproduction. Nonaqueous electrolyte based ultracapacitors are of interestbecause they have much larger decomposition voltage limit compared toaqueous electrolyte systems. The energy that can be stored in anonaqueous ultracapacitor can be greatly increased over that of aqueoussystems. However, organic electrolytes are much less conductive comparedto aqueous electrolytes. This contributes to a relatively highresistance and limits ultracapacitor use for high power applications.Further, high internal resistance causes a high internal loss of storedenergy. This increases the cost of an ultracapacitor because of theconcomitant requirement of enlarged electrode.

It would be advantageous to provide nonaqueous ultracapacitors withreduced internal resistance. A number of solvent systems have beenextensively investigated toward this end. A common system utilizespropylene carbonate (“PC”) for its moderately good conductivity andreasonably wide potential window. PC is the most common solvent used innonaqueous ultracapacitors.

The present invention provides an improved electrolyte compositioncharacterized by increased conductivity and lower resistance which inturn provides performance enhancement and cost reduction in anultracapacitor.

SUMMARY OF THE INVENTION

The invention relates to an ultracapacitor and to a method of making anultracapacitor. The ultracapacitor of the invention comprises two solid,nonporous current collectors, two porous electrodes separating thecollectors, a porous separator between the electrodes and an electrolyteoccupying the pores in the electrodes and separator. The electrolytecomprises (i) a cyclic carbonate solvent and (ii) a cyclic ester solventand an electrolyte salt. The invention also relates to a stack ofultracapacitor cells with an electrolyte comprising (i) a cycliccarbonate solvent and (ii) a cyclic ester solvent and an electrolytesalt.

The method of the invention comprises providing a multilayer structurecomprising two solid, nonporous current collectors, two porouselectrodes separating the current collectors, a porous separator betweenthe electrodes and an electrolyte occupying the pores in the electrodesand separator. The electrolyte comprises (i) a cyclic carbonate solventand (ii) a cyclic ester solvent and an electrolyte salt. The multilayerstructure is sealed to form the ultracapacitor.

In another aspect, the invention relates to a method of making a stackof ultracapacitor cells. In this method, a plurality of bipolar doublelayer ultracapacitor cells is provided in stacked relationship. Anon-porous current collector is provided between each cell with eachcurrent collector having adjoining polarized electrodes of differentcells bonded thereto. The electrodes and separators are saturated withan electrolyte comprising (i) a cyclic carbonate solvent and (ii) acyclic ester solvent and an electrolyte salt. The cells, currentcollectors and separators are sealed to form the stack of ultracapacitorcells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front sectional view of an ultracapacitor;

FIG. 2 is a front sectional view of a series stack of ultracapacitorcells;

FIG. 3 is a cross-sectional view of an exemplary apparatus for sealingan ultracapacitor; and

FIG. 4 is a top cross-sectional view of a separator of a sealedultracapacitor.

DETAILED DESCRIPTION OF THE INVENTION

The electrolyte composition of the invention may be used in a widevariety of ultracapacitors such as described in U.S. Pat. Nos.5,464,453; 5,420,747; 5,150,283; 5,136,472; and 4,803,597; as well asPCT Application WO96/11486 (PCT/US95/12772; Apr. 18, 1996), all of whichare incorporated herein by reference. FIGS. 1 and 2 herein, are based onPCT Application WO 96/11486 and show a non-limiting example of anultracapacitor made by the method of the present invention. In all ofthe Figures of this application, like structures are identified by thesame numbers.

Referring to FIG. 1, ultracapacitor 10 includes a nonconductiveenclosing body 12, a pair of carbon electrodes 14 and 16, an electronicporous separator layer 18, an electrolyte 20, a pair of conductivelayers which are current collectors 22 and 24 and electrical leads 26and 28, extending from the current collectors 22 and 24. One of the pairof current collectors 22 and 24 is attached to the back of eachelectrode 14 and 16. In FIG. 1, electrodes 14 and 16 can each representa plurality of electrodes so long as the electrodes are porous toelectrolyte flow.

The current collectors 22, 24 commonly are made of aluminum because ofits conductivity and cost. In the drawings, the current collectors 22and 24 are thin layers of aluminum foil. However, the electrodes can beany suitable conductive material.

The electronic separator 18 is preferably made from a highly porousmaterial which acts as an electronic insulator between the carbonelectrodes 14 and 16. The separator 18 assures that opposing electrodes14 and 16 are never in contact with one another. Contact betweenelectrodes can result in a short circuit and rapid depletion of thecharges stored in the electrodes. The porous nature of the separator 18allows movement of ions in the electrolyte 20. A wide variety of typesand arrangements of separation layers can be employed, as those ofordinary skill in the electrochemical arts realize. Separation layersare usually made from nonconductive materials such as cellulosicmaterials; glass fiber; polymers such as polyesters or polyolefins; andthe like. In those embodiments in which the separator layers will be incontact with sealant material, they should have a porosity sufficient topermit the passage of sealant and should be resistant to the chemicalcomponents in the sealant. In a typical ultracapacitor, the separatorlayers have a thickness in the range of about 0.5 mil to about 10 mils.Preferred separators 18 are porous polypropylene and tissue cellulosicmaterials. The electrolyte 20 is a composition according to the presentinvention comprising (i) a cyclic carbonate solvent and (ii) a cyclicester solvent and an electrolyte salt. While it is known thatelectrolyte conductivity can be enhanced by the use of mixed solvents,the present invention relates to a particular mixed system, and moreparticularly to a ternary system that exhibits improved performance andlower cost as compared to known mixed solvent systems.

The preferred cyclic carbonates (i) have 5 to 8 carbon atoms. Examplesof the cyclic carbonates include 1,2-butylene carbonate, 2,3-butylenecarbonate, 1,2-pentene carbonate, 2,3-pentene carbonate and propylenecarbonate. Propylene is most preferred.

Preferred cyclic esters (ii) are esters having 3 to 8 carbon atoms.Examples of the cyclic esters include β-butyrolactone, γ-butyrolactone,γ-valerolactone and δ-valerolactone. The most preferred cyclic ester isγ-butyrolactone.

A two solvent component system of the invention comprises (i) the cycliccarbonate solvent and (ii) the cyclic ester solvent in proportions ofabout 1:4 to about 4:1, preferably about 2:1 to about 1:2 and mostpreferably about 1:1.

Further, the electrolyte of the invention can be a three componentsolvent system comprising (i) a cyclic carbonate solvent, (ii) a cyclicester solvent and (iii) at least one additional aprotic organic solventand an electrolyte salt. The at least one additional aprotic organicsolvent (iii) can be a different cyclic carbonate from (i) or adifferent cyclic esters from (ii) as defined above. The at least oneadditional aprotic organic solvent (iii) can also be a chain ether, acyclic ether or a chain carbonate solvent. The preferred chain ethershave 4 to 8 carbon atoms. Examples of the chain ethers includedimethoxyethane, diethoxyethane, methoxyethoxyethane, dibutoxyethane,dimethoxypropane, diethoxypropane and methoxyethoxypropane. Thepreferred cyclic ethers have 3 to 8 carbon atoms. Examples of the cyclicethers include tetrahydofuran, 2-methyl-tetrahydrofuran, 1,3-dioxolan,1,2-dioxolan, 2-methyldioxolan and 4-methyl-dioxolan. Preferably the atleast one additional aprotic organic solvent (iii) is a chain carbonate.The preferred chain carbonates have 3 to 8 carbon atoms. Examples of thechain carbonates include dimethyl carbonate, diethyl carbonate, dipropylcarbonate, methyl ethyl carbonate, methyl propyl carbonate and ethylpropyl carbonate. The most preferred chain carbonate is dimethylcarbonate.

A three component solvent system of the invention comprises (i) thecyclic carbonate solvent, (ii) the cyclic ester solvent and (iii) the atleast one additional aprotic organic solvent in proportions of about4:4:1 to about 1:1:4, preferably about 2:2:1 to about 1:1:2 and mostpreferably about 1:1:1.

Further, the electrolyte solvents of the present invention can includeother solvent components including other aprotic organic solventsincluding the cyclic ester, chain carbonate, cyclic carbonate, chainether and/or cyclic ether solvents aforesaid. Other exemplary organicsolvent components for electrolyte 20 include but are not limited tonitriles such as acetonitrile, acrylonitrile and propionitrile;sulfoxides such as dimethyl, diethyl, ethyl methyl and benzylmethylsulfoxide; amides such as dimethyl formamide and pyrrolidones such asN-methylpyrrolidone.

Suitable electrolyte salts include quaternary ammonium salts such astetraethylammonium tetraflouroborate ((Et)₄NBF₄), hexasubstitutedguanidinium salts such as disclosed in U.S. Pat. No. 5,726,856, thedisclosure of which is incorporated herein by reference, and lithiumsalts such as disclosed by Ue et al., Mobility and Ionic Association ofLithium Salts in a Propylene Carbonate-Ethyl Carbonate Mixed Solvent,Electrochem. Soc., vol. 142, No. 8, August 1995, the disclosure of whichis incorporated herein by reference. The salt can be present in theelectrolyte with a molar concentration of about 0.1 M to about 1.5M,preferably about 0.5M to about 1.5 M, and most preferably about 1.0M toabout 1.5M.

In a preferred embodiment, the electrodes 14, 16 in FIG. 1, are bothcarbon electrodes on aluminum current collectors. The electrode can befabricated by a forming process or by pressing electrode materials in adie and slurry pasting or screen printing carbon as a paste with aliquid phase binder/fluidizer. The liquid phase may be water or anelectrolyte solvent with or without a thinner such as acetone. Both dryand wet electrode formations may include a binder such as polymers,starches, Teflon® particles or Teflon® dispersions in water.

The enclosing body 12 can be any known enclosure means commonly usedwith ultracapacitors. It is an advantage to minimize the weight of thepackaging means to maximize the energy density of the ultracapacitor.Packaged ultracapacitors are typically expected to weigh 1.25 to 2 timesmore than the unpackaged ultracapacitor. The electrical leads 26 and 28extend from the current collectors 22 and 24 through the enclosing body12 and are adapted for connection with an electrical circuit (notshown).

Ultracapacitor 10 of FIG. 1 includes a bipolar double layer cell 30 thatincludes two solid, nonporous current collectors 22, 24, two porouselectrodes 14, 16 separating the current collectors 22, 24 and a porousseparator 18 between the electrodes 14, 16 and an electrolyte 20occupying pores in the electrodes 14, 16 and separator 18. Individualultracapacitor cells can be stacked in series to increase operatingvoltage. The optimum design is to have adjacent cells separated withonly a single current collector. This collector is nonporous so that noelectrolytic solution is shared between cells. This type of design iscalled bipolar and is illustrated in FIG. 2 of the drawings. In abipolar double layer capacitor, one side of the current collectorcontacts a positive electrode and the other side contacts a negativeelectrode of an adjacent cell.

A series stack 40 of the high performance bipolar double layer cells 30(A, B, C and D) is illustrated in FIG. 2. In FIG. 2, each pair ofpolarized carbon electrodes, 14, 16 is separated with a separator 18. Acurrent collector 32 is attached at one surface to charged electrode 14of a first cell. Attached to an opposite surface of the currentcollector 32, is an oppositely charged electrode 16 of a second cell. Ifone side of the current collector 32 is in contact with the negativeelectrode for a first capacitor cell “A,” then the other side of thesame current collector 32 is in contact with a positive electrode for anadjacent cell “B.” A sufficient amount of an electrolyte 20 isintroduced such that the electrolyte 20 saturates the electrodes 14 and16 and separator 18 within each cell. Exterior current collectors 22 and24 are placed at each end of the stack.

The internal current collectors 32 of the series stack of cells arepreferably nonporous layers of aluminum foil designed to separate theelectrolyte 20 between adjacent cells. The exterior current collectorsare also nonporous such that they can be used as part of the externalcapacitor case seal, if necessary. The electronic separator 18 islocated between the opposing carbon electrodes 14 and 16 within aparticular capacitor cell. The electronic separator 18 allows ionicconduction via charged ions in the electrolyte.

The ultracapacitor cell can be constructed by placing the layers ofconductor, electrode and separator along with electrolyte within anenclosing body. The structure can then be subjected to pressure to sealthe layers within the enclosing body. Alternatively, the enclosing bodycan be subjected to pressure and vacuum. The vacuum acts to remove gaseswhile the ultracapacitor is sealed. Alternatively, the ultracapacitorcell can be constructed by providing adhesive between layers andapplying pressure and or heat throughout the adhesive to seal the cell.

FIG. 3 depicts one, non-limiting illustration of an apparatus and methodof sealing an ultracapacitor or series stack of ultracapacitor cellsaccording to the present invention. Referring to FIG. 3, structure 50 isa frame, platform, or other construction but is often a press asdescribed below. An enclosable region is depicted in FIG. 3 as recess52, in which an ultracapacitor series stack 40 is disposed. Theembodiment illustrated in FIG. 3 permits application of vacuum while theultracapacitor is being sealed. Primary vacuum tube 60 communicates withrecess 52. A collapsible membrane 64 can be fastened over theultracapacitor to maintain a vacuum while the cell is being sealed bypressing.

FIG. 3 shows an ultracapacitor cell disposed in the recess area of thepress 50. The cell includes a separator system, comprising an upperseparator layer 42 and a lower separator layer 44. Sealant portions 46and 48 are disposed in a peripheral area between the bottom surface ofseparator 42 and the top surface of separator 44. “Peripheral” refers tothe boundary area of the separator layers. In general, this area shouldbe as small as possible. This boundary area is designated as element 68in FIG. 4. FIG. 4 provides a top, cross-sectional view of a separatorlayer similar to layer 44 of FIG. 3, after sealant has spread to someextent by the action of pressure and, optionally, heat, as describedbelow. The boundary area 68 surrounds the primary section 66 of aseparator layer.

Many different types of sealants can be used in the present inventionand the term is meant to encompass, “glues”, or “pastes.” Sealants aredescribed, for example, in the Kirk-Othmer Encyclopedia of ChemicalTechnology, 3rd Edition, Vol.1, pp.488-508 (1978), and in The CondensedChemical Dictionary, 10th Edition, 1981, Van Nostrand Reinhold Company.In general, the selected sealant should be chemically resistant toelectrolyte. It should also be capable of withstanding operatingtemperatures of the ultracapacitor without substantial degradation.Moreover in those embodiments where the sealant contacts the separators,it should be capable of flowing through the thickness of the separatorlayers. Once cured, the sealant should be substantially impermeable tothe flow or passage of electrolyte.

Heat-curable sealants may be used in some embodiments. Moisture-curedsealants or externally-cured materials may be used. Other embodimentsmay use air-curable or pressure-sensitive sealants, such as “hot melt”glues. Illustrative sealants include those based on acrylic, ethylenesuch as ethylene vinyl acetate (EVA) copolymer, silicone, rubber, epoxymaterials, or combinations of these materials. Commercial examplesinclude the materials commonly referred to as “hot glues.”

The sealants are usually in the form of liquids, pastes, or solids. Asealant may be applied to one or both of the facing surfaces of theseparators or other surfaces. Many techniques are available for applyingsealant. Known application techniques include the use of a spatula,brush, roller, spray, or glue gun. As one example, a bead, strip or“ring” of sealant is applied along the peripheral area 68 of one of theseparator layers. Alternatively, individual droplets of sealant can bedeposited at sites in the peripheral area 68 with the droplets flowingand covering the peripheral area 68 upon the application of pressure,vacuum and/or heat. As yet another alternative, at least one of theseparator layers 18 can be pre-impregnated with sealant. All of thesetechniques cause the sealant to form a continuous layer. In general, theparticular method of deposition is not critical, as long as the sealantis applied to locations where it will eventually form a seal afterpressure or vacuum is released. The ultracapacitor becomes sealed by abarrier which is perpendicular to the horizontal capacitor layers whichare encased in the barrier.

A compressive force is applied to promote the flow of thesealant—especially in the case of sealant compositions with very highsoftening points or glass transition temperatures, such as the EVA basedtypes. Compression can be applied indirectly to the sealant throughupper ultracapacitor layers by means of the mechanical press 50 of FIG.3. Other devices to seal an ultracapacitor include an hydraulic press orpneumatic press or any device for applying compressive force. The press50 of FIG. 3 includes structural frame 70 and adjustable beam 72. Thelength of beam 72 moves in a direction perpendicular to the base portionof the structural frame as controlled by the selective action of handlever 74 and gears 76 and 78. Compression element 80 is detachablyattached as the base of beam 72. Bottom surface 82 can be similar inshape to the peripheral area of the top planar surface of ultracapacitor40. The force applied by the press should be sufficient to cause thesealant to become substantially fluid, to flow and form a continuousbead or strip around the peripheral area of the layer on which it isdeposited. Thus, the particular press force depends in large part on thenature of the sealant. In general, the pressure will be in the range ofabout 1 psi to about 1,000 psi and preferably, in the range of about 10psi to about 100 psi. A lower press force will be suitable for lowerviscosity sealants and a higher press force will be required for higherviscosity materials.

The sealant can be heated while being compressed. Heating enhances theflow characteristics of the sealant. Heating temperature should besufficient to soften the sealant. Preferably, the temperature is highenough to melt the sealant. For a sealant made from an EVA basedmaterial, a suitable temperature will be in the range of about 100° C.to about 300° C.

Heat is applied to the sealant in the press 50 of FIG. 3 by means of astandard electrical heating element that is encased within element 80and is connected to an electrical outlet by way of cord 82. The bottomsurface 84 of element 80 has a shape that aligns with sealant-containingperipheral regions of ultracapacitor 10. Thus, when compression element80 is lowered for compression of the ultracapacitor through membrane 64,heat is transmitted primarily to the sealant containing regions.

A vacuum can be applied to press together the layers of theultracapacitor and to evacuate ambient gasses from the internal regionof the cell structure. In FIG. 3, vacuum tube 60 is connected to avacuum source through vacuum valve 88 with backfill vacuum tube 86. Whenvacuum is applied, the collapsible membrane 64 is positioned over recess52. The membrane 64 maintains the vacuum within the recess and transmitsthe applied compressive force to the layers of the ultracapacitor. Themembrane 64 is heat-resistant to a temperature of about 400° C. Theamount of vacuum applied ranges from about 700 mm mercury to 0.1 mmmercury. A typical vacuum pressure is in the range of about 500 mmmercury to about 0.1 mm mercury.

In operation, the applied vacuum pressure draws collapsible membrane 64tightly against the top of ultracapacitor 10, compressing the individuallayers of the ultracapacitor against platform layer 58 while the actionof compression element 80 presses against sealant-containing regions toinduce sealant 46, 48 to permeate the peripheral regions of separatorlayers 18. The sealant contacts substantially aligned peripheral areas60 of the facing surfaces of conductive layers 22 and 24. As the sealantcures or solidifies, it forms a strong bond to join layers 22 and 24.After sealing is complete, compression element 80 is retracted and theultracapacitor is allowed to cool.

The following example is illustrative of the invention.

EXAMPLE

This example shows that improvement in electrolyte conductivity can beachieved with mixed solvent systems. Conductivity data of 1Mtetraethylammonium tetraflouroborate ((Et)₄NBF₄) were experimentallydeveloped for various solvent combinations and are summarized inTable 1. In the Table PC, GBL, EC, and DMC stand for propylenecarbonate, γ-butyrolactone, ethylene carbonate and dimethyl carbonate,respectively. The ratios in brackets are expressed in volume (ml) exceptfor ethylene carbonate which is expressed in weight (g) since it is asolid at room temperature.

TABLE 1 Electrolyte Conductivity for 1 Molar (Et)₄NBF₄ Solutions at roomtemperature Ratio Ratio Mixture Conductivity, to pure to pure MixtureRatio mS/cm PC GBL PC Pure 12.65 1   0.72 PC + EC 1:1 15.58 1.23 0.88PC + GBL 1:1 15.91 1.26 0.90 GBL + DMC* 1:1 16.00 1.26 0.91 PC + DMC*1:1 16.18 1.28 0.92 GBL Pure 17.63 1.39 1   GBL + EC 1:1 17.95 1.42 1.02GBL + EC + DMC   2:2:1 19.76 1.56 1.12 GBL + EC + DMC   1:1:1 20.16 1.591.14 *Solvents are saturated at 1 Molar or lower (Et)₄NBF₄ at roomtemperature, therefore, data are for saturated solutions. PC = PropyleneCarbonate EC = Ethylene Carbonate (solid at room temperature, but liquidin above solutions) DMC = Dimethyl Carbonate GBL = gamma Butyrolactone

As shown in Table 1, the mixed solvent systems provide more than 50%higher conductivity than conventional PC solvent. Moreover, the ternarysolvent system with γ-butyrolactone, ethylene carbonate and dimethylcarbonate is significantly more conductive than corresponding binarysystems for 1M (Et)₄NBF₄. Thus, the internal resistance of nonaqueousultracapacitors can be greatly reduced, which in turn improves power andenergy performance of ultracapacitor devices and reduces materials cost.

What is claimed is:
 1. An ultracapacitor comprising at least one cell,said cell comprising: two solid, nonporous current collectors, twoporous electrodes separating said current collectors, a porous separatorbetween said electrodes and an electrolyte occupying pores in saidelectrodes and separator, wherein said electrolyte comprises (i) acyclic carbonate solvent and (ii) a cyclic ester solvent and anelectrolyte salt, wherein the electrolyte salt comprises ahexasubstituted guanidinium salt.
 2. The ultracapacitor of claim 1,wherein said electrolyte comprises at least one additional aproticorganic solvent.
 3. The ultracapacitor of claim 2, wherein said at leastone additional aprotic organic solvent comprises a chain carbonate. 4.The ultracapacitor of claim 3, wherein said at least one additionalaprotic organic solvent comprises dimethyl carbonate.
 5. A stack ofultracapacitor cells, comprising at least one of the cells of claim 4.6. A stack of ultracapacitor cells, comprising at least one of the cellsof claim
 3. 7. The ultracapacitor of claim 2, wherein said cycliccarbonate solvent is propylene carbonate, said cyclic ester solvent isγ-butyrolactone, said at least one additional aprotic organic solvent isdimethyl carbonate.
 8. A stack of ultracapacitor cells, comprising atleast one of the cells of claim
 7. 9. A stack of ultracapacitor cells,comprising at least one of the cells of claim
 2. 10. The ultracapacitorof claim 2, wherein a ratio by volume of the cyclic carbonate solvent tothe cyclic ester solvent to the at least one additional aprotic organicsolvent is between 2:2:1 and 1:1:2.
 11. The ultracapacitor of claim 1,wherein said cyclic carbonate solvent comprises propylene carbonate. 12.A stack of ultracapacitor cells, comprising at least one of the cells ofclaim
 11. 13. The ultracapacitor of claim 1, wherein said cyclic estersolvent comprises γ-butyrolactone.
 14. A stack of ultracapacitor cells,comprising at least one of the cells of claim
 13. 15. The ultracapacitorof claim 1, wherein said current collectors comprise an aluminumsubstrate.
 16. A stack of ultracapacitor cells, comprising at least oneof the cells of claim
 15. 17. The ultracapacitor of claim 1, whereinsaid electrodes comprise carbon.
 18. A stack of ultracapacitor cells,comprising at least one of the cells of claim
 17. 19. The ultracapacitorof claim 1, wherein said separator is polypropylene or cellulosic tissuematerial.
 20. A stack of ultracapacitor cells, comprising at least oneof the cells of claim
 19. 21. The ultracapacitor of claim 1, comprisinga plurality of electrodes separating said current collectors.
 22. Astack of ultracapacitor cells, comprising at least one of the cells ofclaim
 21. 23. A stack of ultracapacitor cells, comprising at least oneof the cells of claim
 1. 24. The ultracapacitor of claim 1, wherein aratio by volume of the cyclic carbonate solvent to the cyclic estersolvent is between 2:1 and 1:2.
 25. An ultracapacitor comprising atleast one cell, said cell comprising: two solid, nonporous currentcollectors, two porous electrodes separating said current collectors, aporous separator between said electrodes, an electrolyte occupying poresin said electrodes and a separator, wherein said electrolyte comprisespropylene carbonate, γ-butyrolactone, dimethyl carbonate andtetraethylammonium tetraflouroborate salt.
 26. A stack of ultracapacitorcells, comprising at least one of the cells of claim
 25. 27. A method ofmaking an ultracapacitor, comprising: (A) providing a multilayer cellcomprising two solid, nonporous current collectors; two porouselectrodes separating said current collectors; a porous separatorbetween said electrodes and an electrolyte occupying pores in saidelectrodes and separator, and an electrolyte salt; and (B) sealing saidcell to form said ultracapacitor, wherein said electrolyte comprisespropylene carbonate, γ-butyrolactone, dimethyl carbonate andtetraethylammonium tetraflouroborate salt.
 28. A method of making astack of ultracapacitor cells, comprising: (A) providing in stackedrelationship, a plurality of bipolar double layer ultracapacitor cells,at least one comprising porous, oppositely charged electrodes withionically charged separator disposed between said electrodes; (B)providing a non-porous current collector between each cell with eachcurrent collector having adjoining polarized electrodes of differentcells bonded thereto; (C) saturating said electrodes and separators withan electrolyte comprising a cyclic carbonate solvent and a cyclic estersolvent and a hexasubstituted guanidinium; and (D) sealing said cells,current collectors and separators to form said stack of ultracapacitorcells.
 29. The method of claim 28, wherein said electrolyte comprises atleast one additional aprotic organic solvent.
 30. The method of claim29, wherein said at least one additional aprotic organic solventcomprises a chain carbonate.
 31. The method of claim 29, wherein said atleast one additional aprotic organic solvent comprises dimethylcarbonate.
 32. The method of claim 29, wherein said electrolytecomprises propylene carbonate, γ-butyrolactone, and dimethyl carbonate.33. The method of claim 29, wherein said electrolyte comprises propylenecarbonate, γ-butyrolactone, and dimethyl carbonate.
 34. The method ofclaim 28, wherein said cyclic carbonate solvent comprises propylenecarbonate.
 35. The method of claim 28, wherein said cyclic ester solventcomprises γ-butyrolactone.
 36. A method of making an ultracapacitor,comprising: (A) providing a multilayer cell comprising two solid,nonporous current collectors; two porous electrodes separating saidcurrent collectors; a porous separator between said electrodes and anelectrolyte occupying pores in said electrodes and separator, whereinsaid electrolyte comprises a cyclic carbonate solvent and a cyclic estersolvent and a hexasubstituted guanidinium salt; and (B) sealing saidcell to form said ultracapacitor.
 37. The method of claim 36, whereinsaid electrolyte comprises (iii) at least one additional aprotic organicsolvent.
 38. The method of claim 37, wherein said at least oneadditional aprotic organic solvent comprises a chain carbonate.
 39. Themethod of claim 37, wherein said at least one additional aprotic organicsolvent comprises dimethyl carbonate.
 40. The method of claim 37,wherein a ratio by volume of the cyclic carbonate solvent to the cyclicester solvent to the at least one additional aprotic organic solvent isbetween 2:2:1 and 1:1:2.
 41. The method of claim 36, wherein said cycliccarbonate solvent comprises propylene carbonate.
 42. The method of claim36, wherein said cyclic ester solvent comprises γ-butyrolactone.
 43. Themethod of claim 36, wherein a ratio by volume of the cyclic carbonatesolvent to the cyclic ester solvent is between 2:1 and 1:2.